Secondary coach Todd Bowles has been named as the team's new defensive coordinator.
This comes after the Eagles blew a 10 point lead with just over five minutes to go in Sunday's game against the Detroit Lions.
Castillo was named the Eagles' defensive coordinator last season, prompting criticism for Reid because he had been the team's offensive line coach. The defense struggled early last season and didn't begin to jell until late in Philadelphia's disappointing 8-8 season.
Castillo joined the Eagles in 1995 as an offensive assistant under coach Ray Rhodes. He was promoted to tight ends coach in 1997, and then offensive line coach in 1998.
Bowles was hired by the Eagles in the offseason. He had been on the Dolphins' staff and was named the interim coach last year when Tony Sparano was fired.
The Eagles have a bye this week. In a news release issued Tuesday morning, the Eagles said:
The Philadelphia Eagles today announced they have dismissed defensive coordinator Juan Castillo from his duties with the Philadelphia Eagles and named secondary coach Todd Bowles as the team's new defensive coordinator.
Said Eagles head coach Andy Reid, "I want to make it clear that I have nothing but the ultimate respect for Juan Castillo as a coach and as a person. He's one of the finest football coaches that I have ever worked with. He has served this organization extremely well for 18 years and letting him go was a difficult decision. I know he will continue to be a successful coach in this league and wish he and his family nothing but the best.
"We're six games into the season and average isn't good enough. I know the potential of our team and insist on maximizing it."